Low bend loss single mode optical fiber with chlorine updoped cladding

ABSTRACT

An optical fiber having both low macrobend loss and low microbend loss. The fiber has a central core region, a first (inner) cladding region surrounding the central core region and having an outer radius r 2 &gt;16 microns and relative refractive index Δ 2 , and a second (outer) cladding region surrounding the first cladding region having relative refractive index, Δ 3 , wherein Δ 1 &gt;Δ 3 &gt;Δ 2 . The difference between Δ 3  and Δ 2  is greater than 0.12 percent. The fiber exhibits a 22 m cable cutoff less than or equal to 1260 nm, and r 1 /r 2  is greater or equal to 0.24 and bend loss at 1550 nm for a 15 mm diameter mandrel of less than 0.5 dB/turn.

This is a divisional application of U.S. application Ser. No. 15/264,750filed on Sep. 14, 2016, which claims the benefit of priority to U.S.Provisional Application Ser. No. 62/218,820 filed on Sep. 15, 2015, thecontents of each are relied upon and incorporated herein by reference inits entirety, and the benefit of priority under 35 U.S.C. §120 is herebyclaimed.

FIELD

The present invention relates to optical fibers having low bend losses.

TECHNICAL BACKGROUND

There is a need for low bend loss optical fibers, particularly foroptical fibers utilized in so-called “access” and fiber to the premises(Ft.) optical networks. Optical fiber can be deployed in such networksin a manner which induces bend losses in optical signals transmittedthrough the optical fiber. Some applications that can impose physicaldemands, such as tight bend radii, compression of optical fiber, etc.,that induce bend losses include the deployment of optical fiber inoptical drop cable assemblies, distribution cables with FactoryInstalled Termination Systems (FITS) and slack loops, small bend radiusmultiport located in cabinets that connect feeder and distributioncables, and jumpers in Network Access Points between distribution anddrop cables. It has been difficult in some optical fiber designs toachieve both low bend loss and low cable cutoff wavelength at the sametime.

SUMMARY

Disclosed herein are optical waveguide fibers comprising: (i) a centralcore region having outer radius r₁ and relative refractive index Δ₁; and(ii) a cladding surrounding the central core region and comprising: (a)a first cladding region having an outer radius r₂>10 microns andrelative refractive index Δ₂, and (b) a second cladding regionsurrounding the first cladding region and having relative refractiveindex Δ₃ and an outer radius r₃, wherein the second cladding regioncomprises at least 1.2 wt % chlorine (Cl), and wherein Δ₁>Δ₃>Δ₂, and thedifference between Δ₃ and Δ₂ is greater than 0.12 percent, and Δ₃ isgreater 0.12 (for example, greater than 1.25%); and the fiber exhibits amode field diameter MFD greater than 9 μm at a 1310 nm wavelength andbend loss at 1550 nm for a 15 mm diameter mandrel of less than 0.5dB/turn. In some embodiments r₁/r₂ is greater than or equal to 0.24. Insome embodiments r₁/r₂ is greater than 0.25, more preferably greaterthan 0.3, for example 0.45>r₁/r₂>0.25. In some embodiments0.4≧r₁/r₂≧0.26. According to exemplary embodiments disclosed herein thefirst, first cladding region is comprised of essentially pure silica(SiO₂). In some embodiments the difference between Δ₃ and Δ₂ is greaterthan 0.12 percent, and in some embodiments greater than 0.12 percent(e.g. >0.13%), for example 0.12 to 0.25 percent, or 0.12 to 0.2 percent.In at least some embodiments the fiber exhibits a MAC number >6.5, andin some embodiments greater than 7.5. The exemplary fibers disclosedherein preferably exhibit a 22 m cable cutoff less than or equal to 1260nm, for example 1175 nm to 1255 nm.

According to at least some exemplary embodiments the central core regionof the fiber substantially exhibits an alpha profile with an alpha lessthan 10, for example less than 6 and in some embodiments less than 4.

The fiber designs disclosed herein result in fibers having opticalproperties that are G.652 compliant, MFD between 9 and 9.5 microns at1310 nm, typically between 9.0 and 9.4 microns at 1310 nm, zerodispersion wavelength, λ0, of 1300 nm≦λ0≦1324 nm, cable cutoff less thanor equal to 1260 nm, and attenuation at 1550 nm≦0.189 dB/km. In at leastsome embodiments attenuation is ≦0.185 dB/km, or even ≦0.183 dB/km at1550 nm.

According to the exemplary embodiments described herein the claddingincludes a low index cladding region surrounding the core (also referredto herein as the first cladding region or the inner cladding region).Fibers having this low index cladding region have reduced microbendinglosses. The low index cladding region may have the absolute volume ofbetween about 30 and 90% Δmicrons², for example between 40 and 75%Δmicrons² The low index cladding region may be formed essentially ofpure silica, and is depressed relative to the second cladding regionwhich is updoped with at least 1.2 wt % chlorine.

Preferably, the fibers disclosed herein are capable of exhibiting a wiremesh covered drum microbend loss (i.e., an increase in attenuation fromthe unbent state) at 1550 nm which is less than or equal to 0.07 dB/km,more preferably less than or equal to 0.05 dB/km.

Fibers having a trench region (depressed index region) in the claddinghave improved (reduced) microbending losses. The trench region in thecladding of the fibers disclosed herein may be formed either by downdoping the trench region (e.g. by fluorine (F) doping or by doping withnon-periodic voids) or by updoping the outer cladding region. In otherembodiments, the fibers may include both a trench region and an outercladding region which is updoped with respect to silica (SiO₂), i.e. acladding region which includes an index increasing dopant such asgermania (GeO₂) or chlorine (Cl) in sufficient amounts to appreciablyraise the index of silica.

The embodiments of fibers disclosed herein preferably exhibit a 10 mmdiameter macro bend loss at 1550 nm which is not more than 1 dB/turn(e.g., 0.1 to 0.9 dB/turn). The embodiments of fibers disclosed hereinpreferably exhibit a 15 mm diameter macro bend loss at 1550 nm which isnot more than 0.5 dB/turn (e.g., 0.05 to 0.45 dB/turn). Additionally, atleast some of the embodiments of fibers disclosed herein exhibit a 20 mmdiameter bend loss at 1550 nm which is not more than 0.2 dB/turn (forexample, 0.01 dB/turn to 0.1 dB/turn). At the same time, these fibersare capable of providing an attenuation at 1550 nm which is less than orequal to 0.19 dB/km, more preferably less than 0.186 dB/km, and mostpreferably less than 0.184 dB/km, as well as an attenuation at 1310 nmwhich is less than or equal to 0.34 dB/km, more preferably less than0.32 dB/km. Preferably, according to some embodiments, the 30 mmdiameter bend loss at 1550 nm is not more than 0.02 dB/turn (e.g., 0.002to 0.015 dB/turn). In some embodiments, the 20 mm diameter bend loss at1550 nm is not more than 0.04 dB/turn. In other preferred embodiments,the 20 mm diameter bend loss at 1550 nm is not more than 0.038 dB/turn.In some embodiments, the 30 mm diameter bend loss at 1550 nm is not morethan 0.015 dB/turn.

Some fiber embodiments utilize a primary and a secondary coating,wherein the Young's modulus of the primary coating is less than 5 MPa,more preferably less than 1 MPa, and the Young's modulus of thesecondary coating is greater than 500 MPa, more preferably greater than900 MPa, and even more preferably greater than 1100 MPa.

In some embodiments, the refractive index profile of the optical fiberfurther provides a zero dispersion wavelength of less than 1325 nm. Insome embodiments, the refractive index profile further provides a zerodispersion wavelength of between 1300 and 1325 nm.

Preferably, the refractive index profile further provides a cabledcutoff of less than or equal to 1260 nm, more preferably between 1000 nmand 1260 nm.

In some embodiments, the refractive index profile of the optical fiberfurther provides a mode field diameter at 1310 nm between 9 and 9.5microns.

As used herein, MAC number means mode field diameter at 1310 (nm)divided by 22 m cable cutoff wavelength (nm). In some embodiments, therefractive index profile further provides a MAC number greater than 6.5.In some preferred embodiments, the refractive index profile furtherprovides a MAC number of greater than 7.2, for example greater than 7.2or greater than 7.5, or is at least 8, e.g., 7.2 to 7.8.

According to at least some exemplary embodiments the fiber exhibits awire mesh covered drum microbend loss at 1550 nm which is less than orequal to 0.07 dB/km. According to at least some exemplary embodimentsthe fiber exhibits an attenuation at 1550 nm which is less than or equalto 0.18 dB/km. The exemplary fibers disclosed herein are capable ofexhibiting a wire mesh covered drum microbend loss (i.e., an increase inattenuation from the unbent state) at 1550 nm (WMCD at 1550nm) which isless than or equal to 0.07 dB/km and in some embodiments less than orequal to 0.05 dB/km, such as for example 0.005 to 0.05 dB/km. Theexemplary fibers disclosed herein are capable of exhibiting abasketweave microbend loss at −60° C. (i.e., an increase in attenuationfrom the unbent state) at 1550 nm which is less than or equal to 0.05dB/km, in some embodiments less than or equal to 0.02 dB/km, and in someembodiments less than or equal to 0.01 dB/km such as for example 0.001to 0.01 dB/km.

Reference will now be made in detail to the present preferredembodiments, examples of which are illustrated in the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a refractive index profile corresponding to anembodiment of an optical waveguide fiber disclosed herein;

FIG. 2 illustrates a refractive index profile of another embodiment ofthe optical waveguide fiber;

FIGS. 3 and 4 illustrate two exemplary refractive index profiles offiber embodiments of optical waveguide fibers disclosed herein;

FIG. 5 illustrates an refractive index profile of one exemplarymanufactured fiber embodiment, which shows a central core regionsurrounded by a low refractive index first cladding region and a raisedrefractive index second cladding region;

FIG. 6 illustrates an exemplary refractive index profile of yet anotherexemplary fiber embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Additional features and advantages will be set forth in the detaileddescription which follows and will be apparent to those skilled in theart from the description or recognized by practicing as described in thefollowing description together with the claims and appended drawings.

The “refractive index profile” is the relationship between refractiveindex or relative refractive index and the radial position within thewaveguide fiber. The radius for each segment of the refractive indexprofile is given by the abbreviations r₁, r₂, r₃, r₄, etc. and lower andupper case are used interchangeably herein (e.g., r₁ is equivalent toR₁).

The term “relative refractive index percent” (also referred to herein as“relative refractive index”, and “refractive index delta”) is defined asΔ %=100×(n_(i) ²−n_(c) ²)/2n_(i) ², and as used herein n_(c) is theaverage refractive index of undoped silica. As used herein, the relativerefractive index is represented by Δ and its values are given in unitsof “%”, unless otherwise specified. The terms: delta, Δ, Δ %, % Δ, delta%, % delta and percent delta may be used interchangeability herein. Thatis, as used herein, relative refractive index percent (or relativerefractive index, or refractive index delta) of a given fiber region ismeasured relative to undoped silica. In cases where the refractive indexof a region is less than the average refractive index of undoped silica,the relative index percent is negative and may be referred to as havinga depressed region or depressed index. In cases where the refractiveindex of a region is greater than the average refractive index ofundoped silica, the relative index percent is positive. An “updopant” isherein considered to be a dopant which has a propensity to raise therefractive index relative to pure undoped SiO₂. A “downdopant” is hereinconsidered to be a dopant which has a propensity to lower the refractiveindex relative to pure undoped SiO₂. Examples of updopants include GeO₂(germania), Al₂O₃, P₂O₅, TiO₂, Cl, Br. Examples of down dopants includefluorine and boron.

“Chromatic dispersion”, herein referred to as “dispersion” unlessotherwise noted, of a waveguide fiber is the sum of the materialdispersion, the waveguide dispersion, and the inter-modal dispersion. Inthe case of single mode waveguide fibers the inter-modal dispersion iszero. Zero dispersion wavelength is a wavelength at which the dispersionhas a value of zero. Dispersion slope is the rate of change ofdispersion with respect to wavelength.

“Effective area” is defined as:

A _(eff)=2π(∫f ² r dr)²/(∫f ⁴ r dr),

where the integration limits are 0 to ∞, and f is the transversecomponent of the electric field associated with light propagated in thewaveguide. As used herein, “effective area” or “A_(eff)” refers tooptical effective area at a wavelength of 1550 nm unless otherwisenoted.

The term “a-profile” refers to a relative refractive index profile,expressed in terms of Δ(r) which is in units of “%”, where r is radius,which follows the equation,

Δ(r)=Δ(r _(o))(1−[|r−r _(o)|/(r ₁ −r _(o))]^(α)),

where r_(o) is the point at which Δ(r) is maximum, r₁ is the point atwhich Δ(r) % is zero, and r is in the range r₁≦r≦r_(f) , where Δ isdefined above, r_(i) is the initial point of the α-profile, r_(f) is thefinal point of the α-profile, and α is an exponent which is a realnumber.

The mode field diameter (MFD) is measured using the Peterman II methodwherein, 2w=MFD, and w²=(2∫f² r dr/∫[df/dr]² r dr), the integral limitsbeing 0 to ∞.

The bend resistance of a waveguide fiber can be gauged by inducedattenuation under prescribed test conditions, for example by deployingor wrapping the fiber around a mandrel of a prescribed diameter, e.g.,by wrapping 1 turn around a either a 6 mm, 10 mm, or 20 mm or similardiameter mandrel (e.g. “1×10 mm diameter macrobend loss” or the “1×20 mmdiameter macrobend loss”) and measuring the increase in attenuation perturn.

One type of bend test is the lateral load microbend test. In thisso-called “lateral load” test (LLWM), a prescribed length of waveguidefiber is placed between two flat plates. A #70 wire mesh is attached toone of the plates. A known length of waveguide fiber is sandwichedbetween the plates and a reference attenuation is measured while theplates are pressed together with a force of 30 Newtons. A 70 Newtonforce is then applied to the plates and the increase in attenuation indB/m is measured. The increase in attenuation is the lateral loadattenuation of the waveguide in dB/m at a specified wavelength(typically within the range of 1200-1700 nm, e.g., 1310 nm or 1550 nm or1625 nm).

Another type of bend test is the wire mesh covered drum microbend test(WMCD). In this test, a 400 mm diameter aluminum drum is wrapped withwire mesh. The mesh is wrapped tightly without stretching, and shouldhave no holes, dips, or damage. Wire mesh material specification:McMaster-Carr Supply Company (Cleveland, Ohio), part number 85385T106,corrosion-resistant type 304 stainless steel woven wire cloth, mesh perlinear inch: 165×165, wire diameter: 0.0019″, width opening: 0.0041″,open area %: 44.0. A prescribed length (750 meters) of waveguide fiberis wound at 1 m/s on the wire mesh drum at 0.050 centimeter take-uppitch while applying 80 (+/−1) grams tension. The ends of the prescribedlength of fiber are taped to maintain tension and there are no fibercrossovers. The attenuation of the optical fiber is measured at aspecified wavelength (typically within the range of 1200-1700 nm, e.g.,1310 nm or 1550 nm or 1625 nm); a reference attenuation is measured onthe optical fiber wound on a smooth drum. The increase in attenuation isthe wire mesh covered drum attenuation of the waveguide in dB/km at aspecified wavelength (typically within the range of 1200-1700 nm, e.g.,1310 nm or 1550 nm or 1625 nm).

The “pin array” bend test is used to compare relative resistance ofwaveguide fiber to bending. To perform this test, attenuation loss ismeasured for a waveguide fiber with essentially no induced bending loss.The waveguide fiber is then woven about the pin array and attenuationagain measured. The loss induced by bending is the difference betweenthe two measured attenuations. The pin array is a set of ten cylindricalpins arranged in a single row and held in a fixed vertical position on aflat surface. The pin spacing is 5 mm, center to center. The pindiameter is 0.67 mm. During testing, sufficient tension is applied tomake the waveguide fiber conform to a portion of the pin surface. Theincrease in attenuation is the pin array attenuation in dB of thewaveguide at a specified wavelength (typically within the range of1200-1700 nm, e.g., 1310 nm or 1550 nm or 1625 nm).

Another type of bend test is the basketweave microbend loss test. In thebasketweave microbend loss test, the fibers are wound at high tension ona glass spool and exposed to a temperature cycle. The testing apparatuscomprises of a fixed diameter silica drum. The drum surface is smooth.In this test, the drum diameter is 110 mm. The fiber is wound onto theglass drum with a winding tension of 70 grams, and a pitch of 2 mm(distance between adjacent wraps of fiber). Multiple layers of fiber arewrapped with this tension and pitch. The pitch angles are reversed witheach layer wound. The crossover of the tensioned fibers from theadjacent layers creates the microbend mechanism. A fiber length of 2.5km is used. The initial fiber attenuation measurement is performed atabout 23° , at about 45% RH (relative humidity) with the fiber deployedin the basketweave configuration with 70 grams of tension. Initialattenuation loss measurements are made at wavelengths of 1310 nm, 1550nm, and 1625 nm. An OTDR (optical time domain reflectometer) is used toacquire the attenuation loss data.

After the initial attenuation loss measurement at 23° C., the fiber issubjected to thermal cycling. In the thermal cycling, the fiber is firstcooled from 23° C. to −60° C. at a rate of 1° C./min. The fiber ismaintained at −60° C. for 20 hours and then heated at a rate of 1°C./min back to 23° C. The fiber is maintained at 23° C. for 2 hours,then heated to 70° C. at a rate of 1° C./min and maintained at 70° C.for 20 hours. The fiber is then cooled to 23° C. at a rate of 1° C./minand maintained at 23° C. for two hours. The fiber is then subjected to asecond thermal cycle, which was identical to the first thermalcycle—i.e., it is cooled from 23° C. to −60° C., then heated back to 23°C., maintained at that temperature for 2 hours and then heated from 23°C. to 70° C., after which it is cooled back to 23° C. Finally, aftermaintaining the fiber at a temperature of 23° C. for two hours, afterthe second cycle, the fiber is once again cooled to −60° C. at a rate of1° C./min, held at −60° C. for 20 hours, and then further cooled at arate of 1° C./min to −60° C. The fiber is held at −60° C. for 20 hours,then heated at a rate of 1° C./min back to 23° C. and held at 23° C. for2 hours. The thermal cycling is concluded at this point.

During the thermal cycling of the fiber, the attenuation loss of thefiber is measured continuously. The maximum attenuation loss over thetwo thermal cycles down to −60° C. is determined, and the differencebetween this maximum attenuation loss and the initial attenuation lossat 23° C. is reported herein, as the basketweave microbend loss of thefiber over the temperature range from −60° C. to 70° C. In the thermalcycle down to −60° C., the difference between the attenuation lossmeasured at −60° C. and the initial attenuation loss at 23° C. isreported herein as the basketweave microbend loss of the fiber over thetemperature range from −60° C. to 23° C.

The theoretical fiber cutoff wavelength, or “theoretical fiber cutoff”,or “theoretical cutoff”, for a given mode, is the wavelength above whichguided light cannot propagate in that mode. A mathematical definitioncan be found in Single Mode Fiber Optics, Jeunhomme, pp. 39-44, MarcelDekker, New York, 1990 wherein the theoretical fiber cutoff is describedas the wavelength at which the mode propagation constant becomes equalto the plane wave propagation constant in the outer cladding. Thistheoretical wavelength is appropriate for an infinitely long, perfectlystraight fiber that has no diameter variations.

Fiber cutoff is measured by the standard 2 m fiber cutoff test, FOTP-80(EIA-TIA-455-80), to yield the “fiber cutoff wavelength”, also known asthe “2 m fiber cutoff” or “measured cutoff”. The FOTP-80 standard testis performed to either strip out the higher order modes using acontrolled amount of bending, or to normalize the spectral response ofthe fiber to that of a multimode fiber.

By cabled cutoff wavelength, or “cabled cutoff” as used herein, we meanthe 22 m cabled cutoff test described in the EIA-445 Fiber Optic TestProcedures, which are part of the EIA-TIA Fiber Optics Standards, thatis, the Electronics Industry Alliance-Telecommunications IndustryAssociation Fiber Optics Standards.

Unless otherwise noted herein, optical properties (such as dispersion,dispersion slope, etc.) are reported for the LP01 mode.

Optical fibers (10) disclosed herein are capable of exhibiting aneffective area Aeff at 1550 nm which is greater than about 55 microns²,preferably between 55 and 95 microns², even more preferably betweenabout 65 and 85 microns². In some preferred embodiments, the effectivearea at 1550 nm is between about 75 and 90 micron.

Six exemplary embodiments of optical fiber (10) are shown in FIGS. 1-6respectively. As shown in these figures, optical fiber (10) includes acentral glass core region (1) comprising maximum relative refractiveindex (relative refractive index percent) Δ_(l) and an outer radius r₁.The central core region (1) is surrounded by the cladding region (2′).The cladding (2′) includes a first cladding region (2) and a secondcladding region (3). The first cladding region (2) (also referred hereinas region (2), or the inner cladding region (2)) has a relativerefractive index Δ₂ and an outer radius r₂, wherein 25 microns >r₂>16microns. The first cladding region (2) has an index of refraction thatdepressed relative to, or is lower than the index of refraction of thesecond cladding region (3), such that Δ₃>Δ₂. According to someembodiments, r₁/r₂ is larger than 0.25.

FIGS. 1-6 illustrate that the second cladding region (3) surrounds firstcladding region (2) and comprises a relative refractive index Δ₃. Inexemplary embodiments described herein, Δ₁>Δ₃>Δ₂. In the embodimentsillustrated in FIGS. 1-6, the central core region (1) and claddingregions (2) and (3) are immediately adjacent one another. However, thisis not required, and alternatively additional core or cladding regionsmay be employed. Furthermore, for example, another outer claddingregion, such as a third cladding region (4), may surround the annularsecond cladding region (3). This is shown, for example, in FIG. 6. Thethird cladding region (4), comprises a lower relative refractive indexΔ₄ than the second cladding region (3). I.e., Δ₄<Δ₃, and in someembodiments Δ₄=Δ₂. In some embodiments Δ₂<Δ₄<Δ₃.

Central core region (1) comprises an outer radius r₁ which is defined aswhere a tangent line drawn through maximum slope of the refractive indexof central core region 1 crosses the zero delta line (zero relativerefractive index line). Core region (1) preferably exhibits a relativerefractive index, Δ₁, between about 0.4 and 0.7, and in some embodimentsbetween about 0.42 and 0.6 (relative to pure silica). In someembodiments, Δ_(l) is preferably between 0.44 and 0.55. Core radius r₁is preferably between 3 and 10 microns, more preferably between about4.0 to 7.0 microns, for example 6.0 to 7.0 microns. Central core region(1) may comprise a single segment, step index profile, as shown forexample in FIG. 1. In some embodiments, central core region (1) exhibitsan alpha greater than 0.5 and less than 10, and in some embodiments lessthan 7.5, less than 5, or less than 3. In some embodiments, central coreregion (1) exhibits an alpha greater than 0.5 and less than 10, and insome embodiments less than 7.5, less than 5, or greater than 1 and lessthan 3, and a relative refractive index percent, Δ₁ between 0.38 and0.5. In some embodiments, central core region (1) exhibits an alphagreater than 0.5 and less than 10, and in some embodiments less than7.5, less than 5, or less than 3, and a core region (1) having arelative refractive index percent, Δ₁ between 0.38 to 0.5, and a coreradius between about 4 and 7 microns. In some embodiments, central coreregion (1) substantially exhibits an alpha profile with an alpha greaterthan or equal to 1.5 and less than or equal to 3.

In some embodiments (see, for example, FIGS. 1 and 2), first claddingregion (2) (also referred herein as the inner cladding region (2))surrounds central core region (1) and comprises inner radius r₁ andouter radius r₂;r₁ being defined as above and r₂ being defined as wherethe refractive index profile curve crosses the zero delta line. In somecases the refractive index in the first cladding region (2) isessentially flat; in some embodiments the refractive index of the innercladding region (2) increases as radius increases. In still other casesthere can be fluctuations as a result of small profile design or processvariations. In some embodiments, the first cladding region (2) containsless than 0.02 wt % fluorine. In some embodiments, the first claddingregion (2) comprises silica which is substantially undoped with eitherfluorine or germania, i.e., such that the region is essentially free offluorine and germania. The second cladding region (2) comprises relativerefractive index percent Δ₂ which is calculated using:

Δ₂ = ∫_(r 1)^(r 2)Δ(r)dr/(r₂ − r₁)

The first cladding region (2) preferably exhibits a width between about3 and 13 microns, more preferably between 4 and 12 microns, even morepreferably between about 7 and 9 microns. The outer radius r₂ of theinner cladding region (2) may be greater than 16 microns, and in someembodiments may be greater than 20 microns or even greater than 23microns. In some embodiments, the ratio of the core radius r₁ to theinner cladding region (2) radius r₂ is preferably greater than 0.24, orgreater than 0.3. In some embodiments, r₁/r₂is between 0.24 and 0.55,for example between 0.24 and 4.

The third region of the fiber, or the second cladding region (3),surrounds the lower index inner cladding region (2). The second claddingregion comprises relative refractive index Δ₃ which is higher than therelative refractive index Δ₂ of inner cladding region (2), therebyforming a region which is “updoped” with respect to inner claddingregion 2, by adding at least 1.2 wt % chlorine. Note, however that theinner cladding region (2) may, in addition, be optionally downdopedrelative to pure silica. In some embodiments, there is no fluorine orother down dopants in inner cladding region (2) and the raised indexsecond cladding region (3) (also referred to herein as an outer claddingregion (3)), comprises chlorine doped silica, with or without otherdopants. This is illustrated, for example, in fiber embodiments of FIGS.3, and 4. The outer cladding region (3) comprises a higher refractiveindex than inner the cladding region (2), and preferably comprisesrelative refractive index percent Δ₃ which is greater than 0.12%,preferably at least 0.13%, for example at least 0.14%, relative to puresilica. Preferably, according to some embodiments (see, for example,FIGS. 1-5) the higher relative refractive index of the outer claddingregion (3) (i.e., higher relative to that of the inner cladding region(2)) extends at least to the point where the optical power which wouldbe transmitted through the optical fiber is greater than or equal to 90%of the optical power transmitted. More preferably, the higher relativerefractive index of the outer cladding region (3) extends at least tothe point where the optical power which would be transmitted through theoptical fiber is greater than or equal to 95% of the optical powertransmitted, and most preferably to the point where the optical powerwhich would be transmitted through the optical fiber is greater than orequal to 98% of the optical power transmitted. In many embodiments, thisis achieved by having the “updoped” outer cladding region (3) extends atleast to a radial point of about 30 microns. Consequently, the volume V₂of the inner cladding region (2), is defined herein as being calculatedusing Δ(3-2)(r)rd. between radius r₁ and r₂ , and thus is:

V₂ = 2∫_(r 1)^(r 2)Δ⁽³ ⁻ ²⁾(r)rdr

The volumes V₂ are provided in absolute magnitude (i.e., V₂=|V₂|). Thevolume V₂ of the first cladding region (2) may be greater than 30%Δmicron², and in some embodiments may be greater than 40% Δmicron².According to some embodiments the volume V₂ is between about 30 and 90%Δmicrons², for example between 40 and 75% Δmicrons²

The volume V₂ of the first cladding region (2) is in some embodimentsgreater than 45% Δmicron² and in some embodiments greater than 60%Δmicron². In some embodiments, the volume V₂ of the first claddingregion (2) is less than 90% Δmicron² and in some other embodiments thevolume V₂ of the first cladding region (2) is less than 75% Δmicron².

In some embodiments, the refractive index Δ₃ of the second claddingregion (region (3)) is greater than 0.12 percent, in some embodimentsgreater than 0.13 percent, in some embodiments at least 0.14 percent,and in some embodiments at least 0.15 percent, for example 0.2 percent,when compared to that of the first cladding region (2). In someembodiments, the second cladding region (3) comprises chlorine (Cl) inan amount greater than 12000 ppm, for example at least 12200ppm, or atleast 12500 ppm, or at least 13000 ppm, or 14000 ppm or more, and insome embodiments preferably greater than 15000 ppm, and, in someembodiments, preferably greater than 20000 ppm (2%) by weight (e.g.,22000 ppm, or 25000 ppm, or there between). For example, in someembodiments, the second cladding region (3) comprises chlorine (Cl) inan amount of 12200ppm-25000ppm. Chlorine concentration is describedherein in units of parts per million by weight (abbreviated as ppm wt.or ppm, herein).

The central core region (1) preferably has a positive refractive indexthroughout. The central core region (1) comprises a maximum relativerefractive index Δ1 situated between r=0 and r=7 microns. Δ_(l) isgreater than 0.4%, for example about 0.4 to 0.7%, or 0.42-0.55%. Thatis, the central core region (1) comprises a peak relative refractiveindex that less than or equal to 0.70%.

The inner cladding region (2) preferably has a substantially constantrelative refractive index profile, i.e. the difference between therelative refractive index at any two radii within the inner claddingregion is less than 0.02%, and in some preferred embodiments less than0.01%. Thus, the relative refractive index profile of the inner claddingregion (2) preferably has a substantially flat shape.

The central core region (1) may be a step index core, or as shown forexample in FIGS. 3-5, it may comprise an alpha (α) shape. In someembodiments, r₁ is less than 8.0 microns, and more preferably is between4.0 microns and 7.0 microns. The fibers are capable of exhibiting a bendloss of less than 0.5 dB/turn (at 1550 nm) when wound upon on a 15 mmdiameter mandrel for fibers with MAC numbers greater than 6.5 (e.g. 0.05dB/turn to 0.45 dB/turn at 1550 nm). In some embodiments the fibersexhibit a 20 mm diameter bend loss at 1550 nm of not more than 0.2dB/turn (for example , 0.01 to 0.1 dB/turn. In some embodiments, theoptical fibers disclosed herein have a MAC number of greater than 7, orgreater than 7.5, or even greater than 7.6 or 7.7, or in someembodiments greater than or equal to 8, and a zero dispersion wavelengthof between 1300 and 1324 nm. For example, in some embodiments, opticalfibers (10) have MFD (at 1310 nm) of 9 microns to 9.7 microns, cablecutoff wavelength less than 1.26 μm, MAC numbers between 6.9 and 8.1(e.g., 7.25≧MAC≧8.1), zero dispersion wavelength between 1300 nm and1324 nm and bend loss at 1550 nm around a 15 mm diameter mandrel of lessthan 0.5 dB/turn. In some embodiments, fibers (10) have MFD (at 1310 nm)of 9 microns to 9.7 microns, cable cutoff wavelength less than 1.26 μm,MAC numbers between 6.9 and 8.1 (e.g., 7.25≧MAC≧8.1), zero dispersionwavelength between 1300 nm and 1324 nm and bend loss at 1550 nm around a15 mm diameter mandrel of less than 0.2 dB/turn. According to someexemplary embodiments the cable cutoff wavelength is between 1.175 μmand 1.26 μm, for example cable cutoff wavelength may between 1.175 μmand 1.255 μm, or may be between 1.2 μm and 1.26 μm.

According to some exemplary embodiments the fiber exhibit:

MFD at 1310 nm>9 microns;

Cable Cutoff<1260 nm;

Zero Dispersion Wavelength<1324 nm;

Bend loss at 10 mm diameter<1.5 dB/turn;

Bend loss at 15 mm diameter<0.5 dB/turn dB/turn, wherein the Bend lossis macrobend loss and is measured at a 1550 nm wavelength.

According to some exemplary embodiments the fiber exhibits:

1200<Cable Cutoff<1260 nm;

1300 nm<Zero Dispersion Wavelength<1324 nm;

0.7 dB/turn<macrobend loss at 10 mm bend diameter<1.3 dB/turn;

0.1 dB/turn<macrobend loss at 15 mm bend diameter<0.5 dB/turn;

0.05 dB/turn<macrobend loss at bend 20 mm diameter<0.20 dB/turn;

-   0.1<macrobend loss at 30 mm bend diameter<0.2 dB/turn, wherein the    macrobend loss is measured at a 1550 nm wavelength.

The exemplary fibers disclosed herein are capable of exhibiting a wiremesh covered drum microbend loss (i.e., an increase in attenuation fromthe unbent state) at 1550 nm (WMCD at 1550nm) which is less than orequal to 0.07 dB/km and in some embodiments less than or equal to 0.05dB/km, such as for example 0.005 to 0.05 dB/km. The exemplary fibersdisclosed herein are capable of exhibiting a basketweave microbend lossat −60° C. (i.e., an increase in attenuation from the unbent state) at1550 nm which is less than or equal to 0.05 dB/km, in some embodimentsless than or equal to 0.02 dB/km, and in some embodiments less than orequal to 0.01 dB/km such as for example 0.001 to 0.01 dB/km, and in someembodiments between 0.001 to 0.05 dB/km. The exemplary fibers disclosedherein are capable of exhibiting a basketweave microbend loss at −60° C.(i.e., an increase in attenuation from the unbent state) at 1625 nmwhich is less than or equal to 0.1 dB/km, in some embodiments less thanor equal to 0.05 dB/km, in some embodiments less than or equal to 0.02dB/km, in some embodiments less than or equal to 0.01 dB/km, and in someembodiments between 0.001 to 0.05 dB/km.

The fibers disclosed herein may be drawn from optical fiber preformsmade using conventional manufacturing techniques and using known fiberdraw methods and apparatus, for example as is disclosed in U.S. Pat. No.7,565,820, 5,410,567, 7,832,675, 6,027,062, the specifications of whichis hereby incorporated by reference.

Various exemplary embodiments will be further clarified by the followingexamples. It will be apparent to those skilled in the art that variousmodifications and variations can be made without departing from thespirit or scope of the claims.

Table 1, below, list characteristics of illustrative modeled fiberexamples A-D having a refractive index similar to those shown in FIGS. 3and 4. In particular, set forth below for each example is the relativerefractive index Δ₁, core alpha, and outer radius r₁ of the central core(1), relative refractive index Δ₂ and outer radius r₂ of the firstcladding region (2) and profile volume V₂ of the first cladding region(2), which is calculated between r₁ and r₂, as well as the relativerefractive index Δ₃. Also set forth are chromatic dispersion anddispersion slope at 1310 nm, chromatic dispersion and dispersion slopeat 1550 nm, mode field diameter at 1310 nm and 1550 nm, lateral loadwire mesh microbend at 1550 nm, pin array macrobend at 1550 nm, zerodispersion wavelength, 22 m cable cutoff, MAC number at 1310 nm, and,spectral attenuation at 1310 and 1550 nm, and macro bend induced losses(dB/turn) calculated at 1550 nm wavelength when the bend diameter is 10mm, 15 mm, 20 mm and 30 mm, respectively. In the embodiments of Table 1,the optical fibers exhibits a basketweave microbend loss at −60° C. at1550 nm which is less than or equal to 0.05 dB/km, for example less thanor equal to 0.03 dB/km. The exemplary fibers disclosed herein arecapable of exhibiting a wire mesh covered drum microbend loss (i.e., anincrease in attenuation from the unbent state) at 1550 nm (WMCD at1550nm) which is less than or equal to 0.07 dB/km and in someembodiments less than or equal to 0.05 dB/km, such as for example 0.005to 0.05 dB/km. The exemplary fibers disclosed herein are capable ofexhibiting a basketweave microbend loss at −60° C. (i.e., an increase inattenuation from the unbent state) at 1550 nm which is less than orequal to 0.05 dB/km, in some embodiments less than or equal to 0.02dB/km, and in some embodiments less than or equal to 0.01 dB/km such asfor example 0.001 to 0.01 dB/km.

The specific fiber examples of Table 1 have the first cladding region(2) that are made of pure silica, and the cladding region 3 (i.e., theouter cladding) is silica updoped with greater than 1.2 wt % chlorine(Cl).

TABLE 1 Parameter Example A Example B Example C Example D Core'srelative refractive index, Δ1 (%) 0.48 0.46 0.49 0.5 Core radius, r1(micron) 6.91 6.6 6.6 6.7 Core Alpha 2 2 2 2 Inner cladding radius, r2(micron) 21 21.6 19 19 Core/inner cladding radius ratio, r1/r2 0.33 0.30.35 0.35 Inner cladding's relative refractive index, Δ2 0 0 0 0 (%)Region 3 relative refractive index (outer 0.15 0.13 0.18 0.2 cladding),Δ3 (%) Outer fiber radius, Rmax (microns) 62.5 62.5 62.5 62.5 Clconcentration in region 3, wt % 1.5 wt % 1.3 1.8 2 Inner cladding(trench) volume, V2 (% micron²) 59.4 55.16 57.14 63.22 Zero dispersionwavelength (nm) 1303 1308.3 1304.9 1303 Dispersion at 1310 nm (ps/nm/km)1.37 0.881 1.19 1.36 Dispersion Slope at 1310 nm (ps/nm²/km) 0.09130.0904 0.0906 0.091 Dispersion 1550 nm (ps/nm/km) 19.17 18.63 18.8319.04 Dispersion Slope 1550 nm (ps/nm²/km) 0.062 0.061 0.061 0.061 MFDat 1310 nm (micron) 9.2 9.13 9.01 9.01 MFD at 1550 nm (micron) 10.2810.28 10.06 10.04 LLWM at 1550 nm, dB/m 0.94 1.08 1.64 1.77 Pin Array at1550 nm, dB 37.63 46.66 103.9 123 Cable Cutoff (nm) 1217 1252.45 1242.991259 MAC # (1310 nm MFD/CableCutoff (μm)) 7.56 7.29 7.25 7.16Attenuation at 1550 nm (dB/km) <0.18 <0.18 <0.18 <0.18 Attenuation at1310 nm (dB/km) <0.32 <0.32 <0.32 <0.32 10 mm diameter bend loss, at1550 nm, 0.82 0.4 0.57 0.45 (dB/turn) 15 mm diameter bend loss, at 1550nm, 0.22 0.097 0.147 0.12 (dB/turn) 20 mm diameter bend loss, at 1550nm, 0.06 0.024 0.038 0.032 (dB/turn) 30 mm diameter bend loss, at 1550nm, 0./012 0.003 0.006 0.007 (dB/turn)

Table 2 below lists characteristics of actual manufactured illustrativefiber example E. This fiber also has a primary coating applied theretohaving a Young's modulus of about 0.9 MPa and a secondary coating havinga Young's modulus of about 1200 MPa, and has the refractive indexprofile as shown in FIG. 5. The manufactured fiber corresponding toexample E fiber of Table 2 was drawn from optical preforms on a drawfurnace. In particular, set forth below for each example is the relativerefractive index percent Δ₁, core alpha and outer radius r₁ of thecentral core region (1), relative refractive index percent Δ₂ and outerradius r₂ of the inner cladding region (2); refractive index profilevolume V₂ of the inner cladding region (2), which is calculated betweenr₁ and r₂; and relative refractive index percent Δ₃. Also set forth arechromatic dispersion and dispersion slope at 1310 nm, chromaticdispersion and dispersion slope at 1550 nm, mode field diameter at 1310nm and 1550 nm, lateral load wire mesh microbend at 1550 nm, wire meshcovered drum microbend test at 1550 nm, pin array macrobend at 1550 nm,zero dispersion wavelength, 22 m cable cutoff, MAC number at 1310 nm,1×20 mm diameter bend loss, spectral attenuation at 1310 and 1550 nm,and macro bend induced losses at 1550 nm wavelength when the benddiameter is 10 mm, 15 mm, 20 mm and 30 mm. In this embodiment the firstcladding region was made of silica and the second cladding region wasmade of silica doped with 1.3 wt % Cl.

TABLE 2 Parameter Example E fiber Δ1 (%) 0.47% r1 (micron) 7.5 CoreAlpha 2 r2 (micron) 20.5 r1/r2 0.37 Δ2 (%) 0 Δ3 (%) 0.13 V2 (% micron²)47.3 Dispersion 1310 nm (ps/nm/km) 0.1232 Dispersion Slope 1310 nm(ps/nm²/km) 0.0904 Dispersion 1550 nm (ps/nm/km) 17.76 Dispersion Slope1550 nm (ps/nm²/km) 0.061 MFD 1310 nm (micron) 9.18 MFD 1550 nm (micron)10.16 Lambda 0 (nm) 1308.6 Cable Cutoff (nm) 1208 MAC # (1310 nmMFD/CabCutoff) 7.6 1 × 10 mm bend loss at 1550 nm (dB/turn) 0.8 1 × 15mm bend loss at 1550 nm (dB/turn) 0.104 1 × 20 mm bend loss at 1550 nm(dB/turn) 0.1 1 × 30 mm bend loss at 1550 nm (dB/turn) 0.017 Microbendloss at 1550 nm in 0.005 Basket-weave test at −60 C. for 242 micronscoating diameter (dB/km) Microbend loss at 1625 nm in 0.005 Basket-weavetest at −60 C. for 242 microns coating diameter (dB/km) Microbend lossat 1550 nm in 0.03 Basket-weave test at −60 C. for 200 microns coatingdiameter (dB/km) Microbend loss at 1625 nm in 0.03 Basket-weave test at−60 C. for 200 microns coating diameter (dB/km)

FIG. 5 shows a measured refractive index profile of the manufacturedoptical fiber corresponding to Table 2. In the profile of the exemplaryembodiment shown in FIG. 5, the core region (1) which comprises Δ₁ issurrounded by depressed cladding inner cladding region (2) comprisingΔ₂. Inner cladding region (2) s surrounded by the second (outer)cladding region (3) comprising Δ₃ which is raised—i.e., higher than Δ₂.The difference between Δ₃ and Δ₂ is greater than 0.12% and the fiberexhibits a MAC number >6.5. In the embodiment illustrated in FIG. 5, thefirst cladding region (2) is substantially undoped silica and the secondcladding region (3) is silica doped with chlorine. The optical fiberdisclosed in Table 2 has cladding (2′) that has an outer diameter ofabout 125 micron.

As can be seen in example E fiber of Table 2 above, exemplary fiberembodiments employ a central glass core region having index Δ₁, a firstinner cladding region having index Δ₂, and an outer cladding regionhaving index Δ₃; wherein Δ₁>Δ₃>Δ₂, wherein the difference between Δ₃ andΔ₂ is greater than or equal to 0.12%. Such exemplary fiber embodimentsexhibit a cable cutoff less than or equal to 1260 nm and a bend loss at1550 nm of less than 0.5 dB/turn when wound upon on a 15 mm diametermandrel. These exemplary fiber embodiments also exhibit a mode fielddiameter between about 9 microns and 9.5 microns at 1310 nm, a zerodispersion wavelength between 1300 and 1324 nm, a dispersion slope at1310 nm which is less than or equal to 0.092 ps/nm²/km) These exemplaryfiber embodiments exhibit a Wire Mesh Covered Drum (WMCD) bend loss at1550 nm which is less than or equal to 0.07 dB/km, more preferably lessthan or equal to 0.06 dB/km, and in some embodiments less than or equalto 0.05 dB/km. These exemplary fiber embodiments also exhibit a pinarray bend loss at 1550 nm which is less than 8.5 dB, more preferablyless than 7 dB. These fibers exhibit a Basketweave microbend loss at1550 nm which is less than or equal to 0.05 dB/km, in some embodimentsless than or equal to 0.025 dB/km, and in some embodiments less than orequal to 0.01 dB/km.

In some embodiments, the fibers exhibit a bend loss at 1550 nm, whenwound upon on a 15 mm diameter mandrel, of less than 0.5 dB/turn. Thesefibers also exhibit a bend loss at 1550 nm, when wound upon on a 20 mmdiameter mandrel, of less than 0.1 dB/turn, more preferably less than0.075 dB/turn, and in some fiber embodiments less than 0.05 dB/turn.These fiber embodiments also exhibit a bend loss at 1550 nm, when woundupon on a 30 mm diameter mandrel, of less than 0.025 dB/turn, and insome embodiments of less than 0.003 dB/turn. Some of these examplesemploy chlorine in the second outer cladding region 3 in an amountgreater than 12000 ppm (1.2 wt %), for example between 12000 ppm and25000 ppm. Some of these examples employ chlorine in the outer claddingregion in an amount greater than or equal to 13000 ppm. Some of theseexamples employ chlorine in the outer cladding region in an amountgreater than 13000 ppm and less than 30000 ppm by weight.

Fiber attenuation at 1550 nm is preferably less than 0.20 dB/km, morepreferably less than 0.19 dB/km, even more preferably less than 0.18dB/km. In some preferred embodiments the attenuation at 1550 nm is lessthan or equal to 0.191 dB/km, even more preferably less than or equal to0.188 dB/km, even more preferably less than or equal to 0.185 dB/km,even more preferably less than or equal to 0.182 dB/km, and mostpreferably less than or equal to 0.180 dB/km.

In some embodiments (see, for example, FIG. 6) the second claddingregion (3) is in turn surrounded by a third cladding region (4)comprising Δ₄. In some embodiments cladding region (4) may comprisesilica that is substantially undoped with Ge or Cl (i.e., the relativerefractive index percent of the third cladding region (4) is smallerthan that of the second region (3) (i.e., Δ₄<Δ₃). For example, Δ₄ may benot greater than 0.002% relative to that of pure silica. In someembodiments the third cladding region (4) may be essentially comprisedof silica. In some embodiments, the third cladding region (4) hasviscosity that is larger than the viscosity of the second claddingregion (4) by 0.1×10⁷ Poise at 1650° C. In still other embodiments, theouter cladding region (4) has viscosity that is larger than theviscosity of the second cladding region (3) by 0.1×10⁸ Poise, at 1650°C. In yet other embodiments, region (4) of the outer cladding hasviscosity that is larger than the viscosity of region (3) by 0.1×10⁹Poise at 1650° C. In some embodiments, the third cladding region (4)starts at a radial location larger than 40 microns. In some otherembodiments, the third cladding region (4) starts at a radial locationlarger than 45 microns. In still other embodiments, the third claddingregion (4) starts at a radial location larger than 50 microns. Table 3,below, list characteristics of illustrative modeled fiber examples F-Ghaving a refractive index profile similar to that shown in FIG. 6 andhaving the third cladding region (4) with viscosity higher than that ofthe second cladding region (3).

TABLE 3 Example Parameter Example F G Core's relative refractive index,Δ1 (%) 0.48 0.5 Core radius, r1 (micron) 6.91 6.7 Core Alpha 2 2 Innercladding radius, r2 (micron) 21 19 Core/inner cladding radius ratio,r1/r2 0.33 0.35 Inner cladding relative refractive index, Δ2 (%) 0 0Region 3 relative refractive index (outer 0.15 0.2 cladding), Δ3 (%) Clconcentration in region 3, wt % 1.5 2 Inner cladding (trench) volume, V2(% micron²) 59.4 63.22 Radius of second inner cladding, R3 (microns) 4550 Index of third cladding region 4, Δ4 (%) 0 0 Chlorine in thirdcladding region 4 0 0 Viscosity difference between cladding regions 9.6× 10⁶ 1.24 × 10⁷ 3 and region 4 (Poise) at 1650° C. Outer fiber radius,Rmax (microns) 62.5 62.5 Zero dispersion wavelength (nm) 1303 1303Dispersion at 1310 nm (ps/nm/km) 1.37 1.36 Dispersion Slope at 1310 nm(ps/nm²/km) 0.0913 0.091 Dispersion 1550 nm (ps/nm/km) 19.17 19.04Dispersion Slope 1550 nm (ps/nm²/km) 0.062 0.061 MFD at 1310 nm (micron)9.2 9.01 MFD at 1550 nm (micron) 10.28 10.04 LLWM at 1550 nm, dB/m 0.941.77 Pin Array at 1550 nm, dB 37.63 123 Cable Cutoff (nm) 1217 1259 MAC# (1310 nm MFD/CableCutoff (μm)) 7.56 7.16 Attenuation at 1550 nm(dB/km) <0.18 <0.18 Attenuation at 1310 nm (dB/km) <0.32 <0.32 10 mmdiameter bend loss, at 1550 nm, 0.82 0.45 (dB/turn) 15 mm diameter bendloss, at 1550 nm, 0.22 0.12 (dB/turn) 20 mm diameter bend loss, at 1550nm, 0.06 0.032 (dB/turn) 30 mm diameter bend loss, at 1550 nm, 0./0120.007 (dB/turn)

In the embodiments of Table 3, the optical fibers exhibits a basketweavemicrobend loss at −60° C. at 1550 nm which is less than or equal to 0.05dB/km, for example less than or equal to 0.03 dB/km.

The optical fibers (10) disclosed herein may be surrounded by aprotective coating, e.g., a primary coating P contacting and surroundingthe outer cladding region (3) (or region (4), if the fiber containscladding region (4) surrounding the annular region (3)), the primarycoating P having a Young's modulus of less than 1.0 MPa, preferably lessthan 0.9 MPa, and in some embodiments not more than 0.8 MPa, and in someembodiments not more than 0.5 MPa, and in some embodiments not more than0.3 MPa, for example 0.1 to 1 MPa, and in some embodiments 0.1 to 0.5MPa; and further comprises a secondary coating S contacting andsurrounding the primary coating P, the secondary coating S having aYoung's modulus of greater than 1200 MPa, and in some embodimentsgreater than 1400 MPa, for example at least 1500 MPa, or at least 1600MPa, at least 1800 MPa, or 1400 MPa to 2500 MPa or 1500 MPa to 2500 MPa.The lower modulus of the primary coating (e.g. <0.5 MPa supports goodmicrobend performance, and higher modulus secondary coating (e.g., >1500MPa) supports improve puncture resistance of the secondary coating, evenwhen its thickness is reduced . According to some embodiments the outerdiameter of the secondary coating S is not greater than 250 microns, forexample not greater than 242 microns (e.g., ≦225 microns, ≦210 microns,or ≦200 microns), for example 175-242 microns, or 175 to 225 microns, or180 to 200 microns. The above fiber designs enable good micro and macrobending performance even with coating diameters of less than 225microns, which enables smaller diameter, lower cost, higher fiberdensity cables with excellent optical performance.

As used herein, the Young's modulus, elongation to break, and tensilestrength of a cured polymeric material of a primary coating is measuredusing a tensile testing instrument (e.g., a Sintech MTS Tensile Tester,or an INSTRON Universal Material Test System) on a sample of a materialshaped as a film between about 0.003″ (76 micron) and 0.004″ (102micron) in thickness and about 1.3 cm in width, with a gauge length of5.1 cm, and a test speed of 2.5 cm/min.

Additional description of suitable primary and secondary coatings can befound in PCT Publication W02005/010589 which is incorporated herein byreference in its entirety.

The fibers disclosed herein exhibit low PMD values particularly whenfabricated with OVD processes. Spinning of the optical fiber may alsolower PMD values for the fiber disclosed herein.

It is to be understood that the foregoing description is exemplary onlyand is intended to provide an overview for the understanding of thenature and character of the fibers which are defined by the claims. Theaccompanying drawings are included to provide a further understanding ofthe preferred embodiments and are incorporated and constitute part ofthis specification. The drawings illustrate various features andembodiments which, together with their description, serve to explain theprincipals and operation. It will become apparent to those skilled inthe art that various modifications to the preferred embodiments asdescribed herein can be made without departing from the spirit or scopeof the appended claims.

What is claimed is:
 1. An optical fiber comprising: (i) a central coreregion having outer radius r1 and refractive index Δ1 (ii) a claddingsurrounding the central core region, the cladding comprising: (a) afirst cladding region having an outer radius 25 microns>r2>16 micronsand relative refractive index Δ2, wherein the ratio of r1/r2 is largerthan 0.24 and (b) a second cladding region surrounding the firstcladding region and having a relative refractive index Δ3 and an outerradius r3, wherein the second cladding region comprises at least 1.25 wt% chlorine (Cl), and wherein Δ1>Δ3>Δ2, and wherein the differencebetween Δ3 and Δ2 is greater than 0.12%, and Δ3>0.12%; and said fiberexhibits a bend loss at 1550 nm for a 15 mm diameter mandrel of lessthan 0.5 dB/turn.
 2. The optical fiber of claim 1, wherein thedifference between Δ3 and Δ2 is greater than 0.13%.
 3. The optical fiberof claim 1, wherein the difference between Δ3 and Δ2 is between 0.12%and 0.25%.
 4. The optical fiber of claim 1, wherein said fiber exhibitsa 22 m cable cutoff less than or equal to 1260 nm.
 5. The optical fiberof claim 1, wherein the central core region of said fiber exhibits analpha less than
 10. 6. The optical fiber of claim 1, said fiber furtherexhibiting a wire mesh covered drum microbend loss at 1550 nm which isless than or equal to 0.07 dB/km.
 7. The optical fiber of claim 1,wherein the first cladding region contains less than 0.02 wt % fluorine.8. The optical fiber of claim 1, wherein the first cladding region isessentially free of fluorine and germania.
 9. The optical fiber of claim1, wherein Δ3>Δ2 for a length extending from r2 to a radius of at least30 microns.
 10. The optical fiber of claim 1, wherein the volume of thefirst cladding region is |V2|>30% □micron2.
 11. The fiber of claim 1,wherein said fiber exhibits an attenuation at 1550 nm which is less thanor equal to 0.186 dB/km.
 12. The optical fiber of claim 1, wherein saidcentral core region comprises a maximum relative refractive index (Δ1)of greater than or equal to 0.70%.
 13. The fiber of claim 1, wherein theoptical fiber exhibits: MFD at 1310 nm>9 microns; cable cutoffwavelength<1260 nm; zero dispersion wavelength, □0, of 1300 nm≦□0≦1324nm; bend loss at 10 mm diameter<1.5 dB/turn; bend loss at 15 mmdiameter<0.5 dB/turn wherein the bend loss is macrobend loss and ismeasured at 1550 nm wavelength.
 14. The fiber of claim 1, wherein theoptical fiber exhibits: 1200 nm<cable cutoff wavelength<1260 nm; 1300nm<zero dispersion wavelength<1324 nm; 0.7 dB/turn<macrobend loss at 10mm bend diameter<1.3 dB/turn; 0.1 dB/turn<macrobend loss at 15 mm benddiameter<0.5 dB/turn; 0.05 dB/turn<macrobend loss at bend 20 mmdiameter<0.20 dB/turn; 0.1 dB/turn<macrobend loss at 30 mm benddiameter<0.2 dB/turn; wherein the macrobend loss is measured at 1550 nmwavelength.
 15. The fiber of claim 13, wherein said fiber exhibits awire mesh covered drum microbend loss at 1550 nm which is less than orequal to 0.07 dB/km.
 16. The fiber of claim 13, wherein said fiberexhibits an attenuation at 1550 nm which is less than or equal to 0.18dB/km.
 17. An optical fiber comprising: (i) a central core region havingouter radius r1 and refractive index Δ1 (ii) a cladding surrounding thecentral core region, the cladding comprising: (a) a first claddingregion having an outer radius 25 microns>r2>16 microns and relativerefractive index Δ2, wherein the ratio of r1/r2 is larger than 0.24, and(b) a second cladding region surrounding the first cladding region andhaving a relative refractive index Δ3 and an outer radius r3, whereinthe second cladding region comprises at least 1.2 wt % chlorine (Cl),and wherein Δ1>Δ3>Δ2, and wherein the difference between Δ3 and Δ2 isgreater than 0.12%, and Δ3>0.12%; and said fiber exhibits a bend loss at1550 nm for a 15 mm diameter mandrel of less than 0.5 dB/turn. Furthercomprising a third cladding region surrounding the second claddingregion, the third region having a refractive index Δ4, wherein Δ3>Δ4,and wherein the difference between Δ3 and Δ4 is greater than 0.1%, and−0.02<Δ4<0.02.
 18. The optical fiber of claim 17, wherein the thirdcladding region includes less than 1 wt % of chlorine.
 19. The opticalfiber of claim 17, wherein the third cladding region includes lesschlorine than the second cladding region, or no chlorine.
 20. Theoptical fiber of claim 19 wherein, the third cladding region hasviscosity that is larger than the viscosity of the second claddingregion by at least 0.1×107 Poise.
 21. The optical fiber of claim 1,wherein the central core region of said fiber exhibits an alpha greaterthan or equal to 0.5 and less than or equal to
 10. 22. The optical fiberof claim 17, wherein said fiber exhibits a 22 m cable cutoff less thanor equal to 1260 nm.
 23. The optical fiber comprising: (i) a centralcore region having outer radius r1 and refractive index Δ1 (ii) acladding surrounding the central core region, the cladding comprising:(a) a first cladding region having an outer radius 25 microns>r2>16microns and relative refractive index Δ2, wherein the ratio of r1/r2 islarger than 0.24 and (b) a second cladding region surrounding the firstcladding region and having a relative refractive index Δ3 and an outerradius r3, wherein the second cladding region comprises at least 1.21.25wt % chlorine (Cl), and wherein Δ1>Δ3>Δ2 , and wherein the differencebetween Δ3 and Δ2 is greater than 0.12%, and Δ3>0.12%; and said fiberexhibits a bend loss at 1550 nm for a 15 mm diameter mandrel of lessthan 0.5 dB/turn, said fiber having a coating thereon and exhibiting abasketweave microbend loss at −60° C. at 1550 nm which is less than orequal to 0.05 dB/km.
 24. The optical fiber of claim 22, said fiberfurther exhibiting a basketweave microbend loss at −60° C. at 1550 nmwhich is less than or equal to 0.01 dB/km.
 25. The optical fiber ofclaim 22, said fiber further exhibiting a basketweave microbend loss at−60° C. at 1625 nm which is less than or equal to 0.05 dB/km.
 26. Theoptical fiber of claim 22, said fiber further exhibiting a basketweavemicrobend loss at −60° C. at 1625 nm which is less than or equal to 0.01dB/km.
 27. The optical fiber of claim 22, wherein said coatingcomprises: a primary coating P having a Young's modulus 0.1 to 1 MPa;and a secondary coating S having a Young's modulus of 1400 MPa to 2500MPa, wherein the secondary coating has an outer coating diameter of notgreater than 242 microns.
 28. The optical fiber of claim 22, wherein thesecond cladding region comprises at least 1.3 wt % chlorine (Cl). 29.The optical fiber of claim 1, wherein the volume of the first claddingregion is |V2|>60% % Δmicron2.
 30. The optical fiber of claim 23,wherein said fiber exhibits a 22 m cable cutoff less than or equal to1260 nm.